A memory effect alternating current plasma display panel (PDP) normally comprises two parallel plates enclosing a space containing a discharge gas; between the plates, normally on the internal surfaces of these plates, such a panel has a number of electrode arrays:                normally two arrays of crossed, non-coplanar electrodes, each positioned on a different plate, used for addressing the discharges, and at the intersections of which luminous discharge zones are defined within the space between the plates,        and at least two arrays of parallel coplanar electrodes, positioned on the same plate and used to sustain the discharges; these arrays are coated with a dielectric layer, in particular to add a memory effect; this dielectric layer is itself coated with a secondary protection and electron emission layer, normally based on magnesium oxide.        
Each electrode of one sustain array forms with an electrode of the other sustain array a pair of electrodes delimiting between themselves a succession of luminous discharge zones, normally distributed along a row of discharge zones of the panel. The luminous discharge zones form, on the panel, a two-dimensional matrix; each zone is designed to emit light so that the matrix displays the picture to be viewed.
Normally, one of the coplanar electrode arrays is used for both addressing and sustaining. In this particular case, Yas will be used to denote this electrode array, Ya to denote the second array of coplanar electrodes and X to denote the array of addressing electrodes at right angles to Yas and Ya and positioned on the other plate. The electrode arrays Ya and Yas therefore serve rows of discharge zones, whereas the electrode array X used only for addressing serves columns of discharge zones.
The adjacent discharge zones, at least those that emit different colours, are normally delimited by barriers; these barriers are normally used as spacers between the plates. In the description that follows, each luminous discharge zone of the panel is called a cell.
The walls of the cells are normally partially coated with phosphors sensitive to the ultraviolet radiation of the luminous discharges. Adjacent cells are provided with phosphors emitting different primary colours, so that the combination of three adjacent cells forms a picture element or pixel. In practice, these phosphors cover the sides of the barriers and the plate bearing these barriers, which is normally the plate bearing the electrode array used only for addressing; the addressing electrodes are therefore covered with phosphors.
When the plasma panel is in operation, to display an image, a succession of displays or sub-displays is carried out using the cell matrix; each sub-display generally comprises the following steps:                firstly, a selective addressing step, the purpose of which is to modify the electrical charges on the dielectric layer in each of the cells to be activated, by applying at least one voltage pulse between the addressing electrodes of these cells,        then, a non-selective sustain step during which a succession of voltage pulses is applied between the electrodes of the sustain pairs to provoke a succession of luminous discharges only in the cells that have previously been activated.        
At the end of a sub-display, the cells can be in very different internal electrical voltage states, in particular depending on whether they have been activated in this sub-display; other factors contribute to this spread of internal voltage states, such as the nature of the phosphors corresponding to these cells, the inevitable fluctuations of the dimensional characteristics of these cells, the fluctuations of surface composition of the walls of these cells, which are linked to the panel production methods.
To make the states of the internal voltages of the cells to be addressed uniform, most of the addressing steps are preceded by an equalization step, the main purpose of which is to reset all the cells to be addressed to one and the same internal voltage state, whether or not they have been activated during the preceding sub-display; this equalization, or “reset”, step conventionally comprises an electrical charge-forming operation, or “priming” operation, followed by a charge adjustment operation, also called “erasure” of these charges at the end of which, ideally, the internal voltages within each cell are close to the firing thresholds between addressing electrodes and between sustain electrodes.
For each pair of addressing or sustain electrodes of a cell, an external voltage applied between these electrodes can be associated with an internal voltage in the gas space separating the materials that cover these electrodes. The internal voltage normally differs from the external voltage because of the surface charges that are found on the surface of the insulating materials covering the electrodes, at the interface between these dielectric materials and the gas of the cell. These surface charges result on the one hand from a capacitive effect linked to the dielectric properties of the materials that delimit the cells and on the other hand from an accumulation of “memory” charges produced by the preceding discharges in the gas of these cells.
The internal firing threshold of a cell in a given direction corresponds to a limiting internal voltage value along this direction above which the gas is ionized within this cell. This value depends on the characteristics of the gas in this cell, on those of the materials in contact with the gas in this cell, and on the geometry of the electrodes crossing this cell on the outside of this cell.
In the particular case described previously of three arrays Ya, Yas and X of electrodes, six internal threshold values are normally associated with each cell:                an internal firing threshold between Ya anode and Yas cathode: T[Ya—Yas]        an internal firing threshold between Ya cathode and Yas anode: T[Yas—Ya]        an internal firing threshold between Ya anode and X cathode: T[Ya—X]        an internal firing threshold between Ya cathode and X anode: T[X_Ya]        an internal firing threshold between Yas anode and X cathode: T[Yas—X]        an internal firing threshold between Yas cathode and X anode: T[X_Yas]        
The terms anode and cathode are relative to the internal potentials in the gas of a cell in the vicinity of the electrodes crossing that cell: one electrode is said to be in anode mode relative to another if the potential in its vicinity in the gas is greater than that in the vicinity of the other electrode, this other electrode then being relatively in cathode mode.
The following two internal thresholds have the same value because they characterize discharges in coplanar mode which are generated by electrodes supported by the same plate and normally positioned symmetrically relative to each other:T[Ya—Yas]=T[Yas—Ya]
The following two internal thresholds, which characterize the discharges in matrix mode, therefore between two different plates, are, however, different depending on whether the electrode concerned is acting as an anode or a cathode:T[Ya—X]=T[Yas—X]T[X_Ya]=T[X_Yas]
In practice, when the column addressing electrode X is in cathode mode, the secondary emission of the phosphor covering it being lower than that of the magnesium on the surface of the dielectric covering the row electrode Ya or Yas, the discharges occur at higher voltages than when it is in anode mode.
Normally:                during a priming operation, each electrode of the array Yas used for both addressing and sustain functions is in anode mode relative to the other two electrodes of the arrays Ya and X;        during an erasure operation, with each electrode used for both addressing and sustain functions, Yas is in cathode mode relative to the other two electrodes Ya and X.        
These operations are normally performed by applying a slowly increasing potential difference on the one hand between the two coplanar sustain electrodes and on the other hand between the two matrix addressing electrodes of all the cells of a group to be addressed; the documents FR 2417848 (THOMSON-1978) and U.S. Pat. No. 5,745,086 (PLASMACO-1998) thus describe the application of ramped voltage signals to the electrode or the electrodes used for both addressing and sustain functions while a constant voltage signal is applied to the other addressing-only and sustain-only electrodes.
U.S. Pat. No. 5,745,086 discloses that the reset operations of the cells of a panel are thus performed, advantageously, in each cell, without strong discharge but with a series of “weak” discharges between the electrodes when the slope of the ramp signal applied does not exceed 10 V/μs. These “weak” discharges compensate for the increase in external voltage applied to the electrodes by depositing surface charges on the walls of the cells served by these electrodes, and, since there is no “strong” discharge, the internal voltage in the gas of these cells therefore remains equal to or slightly less than the internal firing threshold defined previously.
The known advantages of reset by weak discharges, also called “positive resistance equalization”, are to enable a precise adjustment of the internal electrical voltages within the cells by producing a weak luminous emission. The precise adjustment is essential to the performance and the effectiveness of the subsequent addressing operation. Limiting this light emission is essential to the contrast performance of the display device.
During the priming operations, electrical discharges occur between the electrodes Yas and X of the cells in the direction X→Yas. These discharges must take place whatever the nature of the phosphor in the cell. However, the phosphors normally have poor secondary emission properties compared to the magnesium oxide that covers the rows. These secondary emission properties are, moreover, highly variable according to the phosphor used. It is, in particular, commonplace for the secondary emission properties of the green phosphors to fall below those of the red and blue phosphors.
The result is that the condition for producing discharges between the electrodes X and Yas in each cell is translated differently in terms of external voltage to be applied according to the nature of the phosphor associated with said cell:                for the phosphors with poor secondary emission coefficient, the threshold voltage is high; therefore, independently of other phenomena, a higher voltage must be applied between the electrodes Yas and X to provoke discharges between these electrodes;        for the phosphors having a better secondary emission coefficient, the voltage to be applied can be weaker.        
Currently, the voltage signals applied, during the priming operation, to the electrodes Yas and X of the cell are independent of the nature of the phosphor of the cells. The same voltage signals are applied to the electrodes X and Yas of the green, red and blue cells. The voltage level of the signals applied is determined to ensure that an adequate electrical charge transfer takes place in all the cells, even the cells for which the phosphor presents a weak secondary emission coefficient. This case is illustrated in FIG. 1. This figure represents the voltage signals applied to the electrodes X and Yas of the cells during the priming phase on the internal walls of the cells. These signals are identical regardless of the phosphor (green, red or blue) of the cell. An increasing voltage ramp is applied to the electrode Yas of the cells. The threshold voltage needed for priming on the internal walls of the cells varies according to the phosphor used. In the example of FIG. 1, the threshold voltage SB of the blue cells is slightly greater than that of the red cells SR. Moreover, the threshold voltage of the green cells SG is very much greater than those of the blue and red cells. The voltage difference between the thresholds of the red and green cells is denoted E1(=SG−SR) and that between the blue and green cells (=SG−SB) is denoted E2. These differences can be as high as 60 volts for E1 and 50 volts for E2. A zero voltage is, moreover, applied to the electrode X of the cells. The priming in the cells is illustrated in FIG. 1 by the presence of the relevant electrical discharges.
Since the same voltage ramp is applied to the blue, red and green cells, discharges occur earlier in the red and blue cells than in the green cells. These discharges continue until the end of the voltage ramp. The result is an excess of discharges in the blue and red cells which unnecessarily increases the background light level (light emitted in the absence of any video content) associated with the priming operation in the cells. This excess of background light is prejudicial to contrast. Furthermore, an excess of discharges between the electrodes X and Yas with the electrode X in cathode mode also represents an additional cause of degradation of the phosphors because, in this discharge mode, it is ions that bombard the phosphors.